This invention relates to an electrophotographic light-sensitive element and a process for the production thereof. More particularly, the invention relates to an electrophotographic light-sensitive element having a photoconductive layer composed of a carbon- and silicon-based amorphous material, and a process for producing such an element.
Electrophotographic light-sensitive elements comprising amorphous Se alone, or amorphous Se doped with an impurity such as As, Te, Sb or Bi, as well as an electrophotographic light-sensitive element comprising CdS, are known. Such light-sensitive elements have several problems. For instance, they are very toxic, and an element comprising amorphous Se is very low in heat stability because amorphous Se crystallizes at temperatures of about 100.degree. C. or more. Furthermore, the light-sensitive film has low mechanical strength and low resistance to hydraulic shock.
Recently, techniques have been proposed to overcome these problems of such conventional electrophotographic light-sensitive elements by using a photoconductive layer composed of amorphous silicon. Amorphous silicon prepared by hydrogen-free vapor deposition or sputtering is not desirable for use in an electrophotographic light-sensitive element, because its dark specific resistance is as low as 10.sup.5 ohm.cm, and its photoconductivity is also very low. This is believed to be due to many defects in the structure, caused by many broken Si-Si bonds; the hopping conduction of thermally excited carriers owing to the high average density of localized states (10.sup.20 /cm.sup.3) within an energy gap between the conduction and filled bands of the silicon is the cause of low dark specific resistance and the capture of light-excited carriers is the cause of poor photoconductivity.
It is reported in Advances in Physics, Vol. 26, No. 6, p. 312 ff., 1977, that a non-doped amorphous silicon prepared by the glow discharge decomposition of silane (SiH.sub.4) gas has a dark specific resistance of 10.sup.9 to 10.sup.10 ohm.cm. "Solid State Communications," Vol. 20, p. 969 ff., 1976 reports that an amorphous silicon prepared by reacting silicon with hydrogen by means of high-frequency sputtering has a dark specific resistance of 10.sup.9 ohm.cm. As reported therein, hydrogen compensates for the defects in the silicon crystal structure, and reduces the average density of localized states within an energy gap between the conduction and filled bands of the silicon to as low as 10.sup.17 to 10.sup.18 /cm.sup.3. The thus-produced silicon has very good photoconductivity, and valence electron control for providing a p- or n-type semiconductor is possible. However, to provide an electrophotographic light-sensitive element having an invariably high dark specific resistance is difficult without controlling the formation conditions very strictly.
Another amorphous silicon carbide produced by glow discharge decomposition is reported in Philosophical Magazine, Vol. 35, p. 1 ff., 1977, and an amorphous silicon carbide produced by high-frequency sputtering is reported in Thin Solid Films, Vol. 2, p. 79 ff., 1968. The carbide described in the first report has a dark specific resistance at room temperature of at least 10.sup.12 ohm.cm, and the one described in the second report has a dark specific resistance at room temperature of 10.sup.8 ohm.cm. However, few studies have been made on the photoconductivity of the amorphous silicon carbide.